This invention relates to processes for removing and recovering sulfur dioxide and nitrogen oxides from gas mixtures containing them and especially from mixtures having low concentrations of these substances. Such mixtures include combustion gases produced in burning coal, oil, and other low molecular weight hydrocarbons in power plants, industrial furnaces and the like. Lower valence nitrogen oxides such as nitric oxide (NO) are extremely difficult to remove from such mixtures. However, nitrogen oxides are regarded as air pollutants and many nations now mandate reducing their emission into the atmosphere. This invention not only permits compliance with such mandates, but permits recovery of these pollutants as commercially valuable products in such quantity and quality that air pollution control may become profitable at best, or at least far less costly.
Where a gas mixture includes both sulfur dioxide and lower valence nitrogen oxides, and removal of both is desirable, these new processes for removing and recovering sulfur dioxide and lower valence nitrogen oxides may be combined into a single step, or may be effected sequentially.
The most common oxide of nitrogen in combustion gases is the lower valence nitric oxide (NO), which generally exceeds the concentration of nitrogen dioxide by a factor of about 10. In these combustion gas mixtures, lower valence oxides of nitrogen may be present in concentrations in the range of about 200 to about 20,000 parts per million. But since these mixtures are evolved at such rapid rates, for example, about 1,000,000 cubic feet per minute from a 500 megawatt power plant, the quantity emitted to the atmosphere is very large and creates a serious air pollution problem. The processes of this invention are especially applicable in minimizing these emissions, or at least lowering them to, say, about 50 parts per million, without seriously impeding the high flow rate of the gas stream from the power plant to the atmosphere.
The conversion of lower valence nitrogen oxides to higher valence nitrogen oxides to facilitate their removal from a gas mixture is, broadly, not an entirely new concept. Thus, for example, U.S. Pat. Nos. 1,420,477; 3,733,393, and 3,927,177 propose oxidizing oxides of nitrogen to remove them more easily from gaseous streams. All of these primarily seek simple removal of the pollutants and are costly to operate. None discloses removing pollutant nitrogen oxides or sulfur dioxide or both from combustion gases and recovering them as useful products in a practical and economic manner. Combustion gases from the burning of carbonaceous fuels also include substantial quantities of carbon dioxide. These quantities may be in the range of about 5% to about 20% by volume. Although emission of carbon dioxide to the atmosphere is not presently under severe attack as an environmental problem, capture and recovery of carbon dioxide may be highly desirable to provide raw materials for other commercially valuable products. The recovery techniques that form important parts of the processes of this invention permit recovery of large volumes of carbon dioxide in high purity at low cost, thus contributing to the overall efficiency and economy of these processes.